Rendering in GPU and CPU in different instances at once
Yes, mixing CUP and GPU rendering on two instances should be faster if you have enough memory to fit two instances of the rendering job. Just make sure you set the CPU rendering instance to a lower priority (using the task manager on Windows, or nice with Linux) so that it doesn't take any CPU resources away from the GPU instance. Otherwise the GPU instance might be starved for instruction from the CPU and cause a slowdown.

With Blender 2.79a, a built-in mixed CPU/GPU rendering mode is also available. Just need to enable it from the User Preferences: